The determination of whole blood analytes is an established diagnostic tool in the medical and health care industries. One problem encountered in the determination of whole blood analytes is the penetration or permeation of red blood cells (RBC) into the reactive, analyte determining sites in the testing system. A major cause of immuno assay or chemical assay interference in the detection of a signal from an analyte detection zone arises from RBC in the test fluid. As the test fluid contacts an absorbent detection zone on a membrane carrier, the RBC along with the other components of the test fluid are absorbed into and penetrate through the membrane and become intermingled in the detection zone. The presence of whole RBC in the detection zone results in a discoloration which physically and chemically interferes with colorimetric assay procedures.
A red blood cell is known to comprise an outer membrane enclosing a solution that is high in concentration of hemoglobin. The red blood cell and the free hemoglobin from hemolysis of the cell can impart a color to the detection zone ranging from light pink to dark maroon. As a consequence, the production of a visual chemical signal can be partially or wholly obscured by the presence of the hemoglobin color in the detection zone. Furthermore, the hemoglobin can block the production of electromagnetic radiation in a fluorescent-type signal generating indicator system. The rapidity of use, accuracy and precision of the dry test strip in the qualitative or quantitative analysis of analytes can be seriously inhibited by the presence of RBC, hemoglobin and other contents of the red blood cell in the detection zone or layer.
Various techniques have been developed to eliminate the physical and chemical interferences created by the presence of whole RBC in the detection zones. One alternative, has been the physical elimination of the RBC from the sample. Specifically, centrifuge techniques have been used to spin down samples thereby expediting the removal of RBC from the samples. Agglutinating agents have also been used to clump RBC and facilitate physical collection and removal of whole RBC from a sample. Alternatively, autonomous or spray applied size exclusion membranes having a definite pore size have been used to create and allow analyte penetration of the detection zone but exclude whole RBC from the detection zone membrane.
However, the use of size exclusion processes does not completely eliminate interferences created by the presence of RBC at the detection zones. A red blood cell can squeeze through a pore having a smaller relative diameter than that of the red blood cell due to the malleable or flexible character of the cell. Furthermore, the use of smaller pores reduces the real volume of analyte which is allowed to pass through any size exclusive membrane and actually contact the detection zone. Consequently, the reduced flow of analyte to the detection zone may result in variable assay results which prove to be undependable in any given instance.
Accordingly, a need exists for a test system for determining select analytes in whole blood samples which is unaffected by the chemical or physical interferences normally created by RBC.